1. Field of the Invention
The present invention generally relates to heart monitoring, and in particular to methods and devices for monitoring the mechanical properties of the heart.
2. Description of the Prior Art
The heart is an essential organ in humans and most animals, pumping blood throughout the human/animal body. As a consequence, it is fundamentally important that the mechanical pumping properties of the heart operate correctly.
There are several diseases and conditions that negatively affect these mechanical properties of the heart, including cardiac ischemia. Cardiac ischemia is a condition where the flow of oxygen-rich blood to the heart muscle is restricted. This form of ischemia occurs when an artery becomes narrowed or blocked for a short time, preventing oxygen-rich blood from reaching the heart. If ischemia is severe or lasts too long, it can cause a heart attack (myocardial infarction) and can lead to heart tissue death. In most cases, a temporary blood shortage to the heart causes pain of angina pectoris. This will then be an indication to the patient that something is not right and he/she should contact a physician.
However, in other cases, there is no pain. These cases are called silent ischemia in the art. This is a severe condition as the patient may not notice that an ischemic condition has occurred or is present and will therefore not contact a physician.
The American Heart Association estimates that 3 to 4 million Americans have episodes of silent ischemia. People who have had previous heart attacks or those who have diabetes are especially at risk for developing silent ischemia. Heart muscle disease (cardiomyopathy) caused by silent ischemia is among the more common causes of heart failure in the United States.
Today there are two main tests that can be used to diagnose silent ischemia. Firstly, an exercise stress test can show blood flow through the coronary arteries in response to exercise, usually walking on a treadmill. Secondly, Holter monitoring records the heart rate and rhythm over at least a 24-hour period. The patient wears a recording device, the Holter monitor, which is connected to disks on the chest. A physician can then look at the printout of the recording to find out if the patient has had episodes of silent ischemia while he/she was wearing the Holter monitor.
However, these two tests require the active participation of a physician. The Holter monitoring is further limited as episodes of silent ischemia must occur during the actual monitoring and previous episodes of ischemia may remain unnoticed. In addition, it is highly unlikely that a patient suffering from attacks of silent ischemia indeed will contact a physician and undergo one of the prior art tests. As silent ischemia seldom has any symptoms, the patient is not aware of the deleterious condition and will therefore not visit a physician.
There are also other conditions and diseases that affect the mechanical properties of the heart in addition to ischemia. For example, heart infarcts cause necrosis of the heart tissue, poor inter-chamber synchronization will cause remodulation while other heart failures can lead to increased heart size (hypertrophy).
The document EP 1 384 433 discloses an apparatus for early detection of an ischemic heart disease. The apparatus comprises means for measuring an intracardiac impedance of a patient's heart and generating a corresponding impedance signal. A notch detector is provided for detecting the occurrence of a notch in the impedance signal coincident with the entry of blood into the ventricle. The apparatus also comprises a pattern recognition means for comparing the measured post-notch impedance curve with a stored predetermined reference impedance curve template to detect an ischemic heart disease.
There is therefore a need for a monitoring of the mechanical properties of the heart that can be used for detecting ischemia or another heart disease or condition affecting these properties. In particular, such monitoring should be able to also detect of silent ischemia and other unnoticed heart failures.